Enveloped RNA viruses are the etiological agents for many human diseases-ranging from yellow fever, the first human disease attributable to a virus (1902), to acquired immune deficiency syndrome, caused by the retrovirus HTLVIII/LAV (1984). Although these two viruses, as well as other enveloped RNA viruses, differ importantly in their genomic structure, composition and mechanisms for gene expression, they have some activities in common in their replication cycle and all synthesize virus-specific transmembranal glycoproteins that interact with virus nucleoprotein structures during virus assembly. Experiments in this proposal are focused on this common activity but employ a simple "model" enveloped RNA virus-cell system, Sindbis virus replication in tissue culture cells. These projects constitute an ongoing research program whose goal is the elucidation of critical biochemical events in glycoprotein processing and assembly of enveloped RNA viruses. The work proposed here will utilize recombinant DNA technology to obtain site-specific mutations in a specific region of this virus' glycoprotein to determine what factors are required to achieve the highly selective interaction between the transmembranal virus "spike" proteins and nucleocapsid of the virus. Systems to measure binding between nucleocapsids and glycoproteins will also be sought. Of particular interest is what role fatty acylation, a modification present in many virus and cell glycoproteins, may play in virus assembly. In vitro mutagenesis will be used further to analyze a small region in the Sindbis genome required for processing and insertion of glycoprotein into the cell's membrane. Our ability to detect effects of directed glycoprotein changes on virus assembly are now feasible because a cDNA capable of transcription into an infectious RNA has been developed.